High frequency passive radio range finder

ABSTRACT

The application discloses an invention for determining the direction and distance of a remotely located radio source based on the principle that the distance to a station is inversely proportional to the root mean square value or peak value of the received continuous bearing signal as a function of time which has first been passed through a twenty minute averaging or smoothing window.

Unite States Patent 1 [111 3,798,647

Bailey Mar. 19, 1974 HIGH FREQUENCY PASSIVE RADIO 3.307.192 2/1967Attwood 343/112 D RANGE FINDER 3,476,921 11/1969 Jones et al. 1. 343/112D [75] Inventor: Albert 1). Bailey, Champaign, Ill.

[73] Assignee: The United States of America as represented by Secretaryof the Navy, Washington, DC.

[22] Filed: May 25, 1972 [21] Appl. No.: 256,961

Primary Examiner-Maynard R. Wilbur Assistant Examiner-T. M. BlumAttorney, Agent, or Firm-R. S. Sciascia; R. J. Miller [57] ABSTRACT Theapplication discloses an invention for determining I I the direction anddistance of a remotely located radio fihSbCll Source based on theprinciple that the distance to a [58] Fie'ld D 100 R station isinversely proportional to the root mean 343/1 13 square value or peakvalue of the received continuous bearing signal as a function of timewhich has first 56] References Cited been passed through a twenty minuteaveraging or UNITED STATES PATENTS Smoothing wmdow' 3,577,146 5/1971Freier 343/112 D 2 Claims, 3 Drawing Figures R F SIGN AL 3 32 1 3 3RADIO LOW PASS RMS or PEAK 7- DIRECTION FILTER Q READING 30 FINDER 20MINUTE VOLT METER MOST CRITERIA RMSor PEAK PROBABLE TEQTING DEVIATIONRANG'E [)EVICE PMENIEDHAR l 9 m4 3; 798L647 ONOSPHERE -I4 TRANSMITTERREcEIvER 22 RMS or PEAK DEVIATION RF sI A 6N 3|, 32E 33 RADIO LOW PASSRMSorPEAK I DIRECTION FILTER -R READING 30 FINDER 20 MINUTE VOLTMETERMOST CRITERIA RMSorPEAK PROBABLE TESTING DEVIATION RANGE DEVICE HIGHFREQUENCY PASSIVE RADIO RANGE FINDER For many years the radio directionfinding community has been interested in passive radio directionlocation where the azimuthal direction of arrived data from two or moredirections was known. Direction findings are combined to determine themost probable point of origin of the radio source.

It is well known that radio signals from a station will travel in a lineof sight mode and those which encounter the ionosphere at some criticalangle will be reflected back down to the earth and will be received at aremotely located station. Those signals that encounter the ionosphere ata higher angle will pass therethru. A passive listening stationreceiving a signal from a remotely located station may transmit a signalhaving a frequency close to that of the received signal and along thesame angles of arrival and by measuring the backscatter from theionosphere may determine the distance to the reflecting point and bytiming the echo or echo signal estimate the range; this would be anactive system.

In this application an observed phenomenon is used in conjunction .withexisting equipment to utilize a newly recognized principle for passiveradio direction locating devices.

That principle is that either the root mean square of the receivedbearing signal from a remote source or the peak signal received fromthat source when averaged over a period ideally some minutes long isinversely related to the range.

The invention provides several advantages over existing State of the Artpassive radio direction finders. Among those is that this invention maybe utilized with existing radio direction finder facilities and may beincorporated therein without modification of existing facilities.Further, it provides an additional statistical measure that hassignificance in estimating the range to the source of an unknown signal.This invention is simple to implement and construct with attendant lowcost which would augment its availability for utilization.

Therefore, it is an objective of this invention to pro vide an improvedradio direction finder.

It is yet a further objective of this invention to provide an improvedradio range finder capable of being coupled to an existing radiofrequency direction finder of the passive type including the addition ofa very low pass filter; a root mean square or peak reading digitalvoltmeter coupled to the filter, means coupled to the voltmeter fordetermining the root mean square or peak deviation and means including acriteria test device whereby the most probable range to a radiotransmitter may be estimated by comparing inversely root mean square orpeak deviation with the range.

It is yet a further objective of this invention to provide an improvedpassive type high frequency radio direction finder whereby the low passfilter passes signals having 15 to 30 minute cycle or period.

It is yet a further objective of this invention to provide a passivetype radio frequency range finder particularly adapted to Wullenweber orCDAA type of radio direction finder equipment for passively determiningthe range of a received signal.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 shows the surface of the earth having transmitting and receivingstations located thereon.

FIG. 2 is a graph showing inverse relationship between root mean squarevalue or peak value and range.

FIG. 3 shows in block diagram form the invention and equipment necessaryto practice it.

It is well known that the earths curvature 10 depicted in FIG. 1 is suchthat radio stations remotely located from each other receive signalsonly thru the bounce" from the ionosphere ll. As shown, transmitter 12projects a signal along path D to the ionosphere 11.

Since the angle of incidence of the signal at the ionosphere is belowthe critical angle, the signal is reflected along path D to a passiveradio direction finder 14. The distance or range over the surface of theearth between the transmitter and receiver is designated as R which isthe value to be determined. The ionosphere is not a small mirror likereflecting surface which gives precise figures, but rather is afluctuating, changing region that produces difficulties in ranging. Ithas been observed that certain fluctuation and variations of theionosphere layer tend to repeat themselves over some 20 minute intervalsand that this is very repeatable.

If the directional signal of the receiver is taken over this 20 minuteperiod and averaged out to obtain the root mean square value of thesignal or the peak deviation of the signal it will be seen that therange or distance is inversely proportional to either value. This isshown in FIG. 2 by curve 21. Wherein the vertical axis 22 the scale isthe magnitude of mean root square or peak deviation and the horizontalaxis 23 scale is range to the transmitter. In a specific situation thevertical intercepts at 24 projected along the horizontal line 25 to theintersection 26 with line 21 and dropped to line 23 at point 28 willgive the most probable range to the target.

FIG. 3 in block diagram form shows the invention as utilized. Anincoming radio frequency signal 30 is received by existing radiodirection finding equipment 31 which may be responsive to radiofrequency signals in the 3 to 30 megahertz range, and is coupled to lowpass filter 32. This filter has a 20 minute window which means for thatperiod of received signals the filter is open and this is repeated.

This filter is a digital device whose output is coupled to the digitalvoltmeter 33 where the root mean square value or peak reading isobtained as shown by block 34. This reading is coupled to the criteriatest device 35 which would be predetermined curves for this relationshipas shown in FIG. 2 and most probable range is thereby determined asshown by block 36.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. An improved passive type high frequency radio range findercomprising:

a. A low pass filter having a pass band capable of receiving radiosignals in the 3 to 30 mega hertz range that fluctuate about a 20 minuteperiod;

root mean square or peak deviation to the range chart.

2. The improved passive type high frequency radio direction finder ofclaim 1 wherein the input to said filter is coupled to the radiofrequency output of conventional radio direction finding equipment.

1. An improved passive type high frequency radio range findercomprising: a. A low pass filter having a pass band capable of receivingradio signals in the 3 to 30 mega hertz range that fluctuate about a 20minute period; b. A root mean square or peak reading digital voltmetercoupled to said filter, to measure the strength 3 to to mega hertzsignal over said period; c. Means coupled to said voltmeter fordetermining the root mean square or peak deviation, and; d. Meansincluding a criteria testing device whereby a most probable range to aradio trasmitter estimate is obtained by comparing invErsely either theroot mean square or peak deviation to the range chart.
 2. The improvedpassive type high frequency radio direction finder of claim 1 whereinthe input to said filter is coupled to the radio frequency output ofconventional radio direction finding equipment.